Hydraulic fracturing (fracturing) is widely used for stimulating petroleum production and recovery from subterranean formations.
It involves the injection of a suitable fluid down a well bore to reach a formation; the fluid shall be injected under sufficient pressure to extensively crack the formation and to provide passageways for the oil and gas that are contained in the pore spaces of the formation and help them flowing to the well bore. Suitable particulate materials (proppants) are often injected in the formation to prevent the closure of the fractures once pressure is released. Usually, fracturing fluids are gelled with water soluble polymers, especially with natural polymers or natural polymers derivatives, to most effectively widen the fractures and inhibit the fluid loss.
To provide a gelled fracturing fluid, these water soluble polymers shall be previously dissolved in the aqueous component of the fluid and then gelled with a crosslinking composition which is generally based on titanium, zirconium, aluminum or boron salts.
Most often, boron salts are the crosslinking compositions of choice because of their low cost and efficiency.
Nonetheless, the release of boron salts in the environment tends to be discouraged because, unfortunately, boric acid derivatives revealed to be toxic for reproduction.
Moreover, boron salts cannot be used above 150° C., because above this temperature their gels are unstable. At higher temperature, expensive and pH sensitive zirconium salts shall be used.
Therefore, it would be highly desirable to provide a gelling composition for aqueous based fracturing fluids which is devoid of boron salts, utilizable in a wide range of pH and temperature, is cheap and little sensitive to salts. From U.S. Pat. No. 7,987,912 (Schlumberger Techn. Corp.), a viscosifier for oilfield fluids is known that is the combination of a polymer and crosslinking agents that undergoes the Diels Alder reversible reaction to form a gel. The thermal conditions at the subterranean location should favour both the forward reaction (gelling) and reverse reaction (breaking). The critical temperatures of the forward and reverse reaction can be tuned with various methods, such as adding a competitive compounds or a catalyst.
U.S. Pat. No. 7,858,561 (Schlumberger Techn. Corp.) describes a viscosifying agent for wellbore fluids including a polymer polymerized using at least two pre-polymers “A” and “B”, the polymer being capable of essentially fully degrading into soluble fragments; the synthesis of an A-B polymer obtained by radically grafting a diethylenically unsaturated polymeric compound (B) on a low molecular weight non-modified guar(A) in solution is there reported.
US 2013/0312970 (Schlumberger Techn. Corp) discloses a method for viscosifying aqueous fluids by using polymers that undergo crosslinking in the presence of functionalized nanoparticles. In one embodiment, the functionalized nanoparticles bear boronic acid groups and the polymer is guar. The method is said to allow minimizing the use of boron with a low concentration of the polymer.
Synthesis of double bond modified polysaccharides and crosslinking of double bond modified polysaccharides are also known in the literature.
By way of example, the aim of the study published in J. of App. Polym. Sci. 117, 148-154 (2010) is to synthesize an acryloyloxy guar gum via a Schotten-Baumann reaction in aqueous media. The reaction products were characterized using FTIR, 13C-NMR, wide angle X-ray diffraction techniques to ascertain the effect of acrylation on the structure of guar gum. The acrylation of guar gum was found to be limited to the primary hydroxyl groups on the guar gum molecule. The maximum degree of substitution (DS) was found to be 0.56, which was observed after 3 h of reaction.
In Acta Biomaterialia 5, 3441-3452 (2009) it is reported that very low molecular weight guar gum was modified with glycidyl methacrylate (GMA) in aqueous solution to produce a series of water-soluble photopolymerizable guar gum-methacrylate (GG-MA) macromonomers of different molecular weights. The effects of the molecular weight of the GG-MA macromonomers and of the degree of methacrylation on the properties of GG-MA hydrogels were studied.
EP 2532682 (BASF) describes a process for the preparation of polysaccharide esters of ethylenically unsaturated acids. The esterification of polysaccharides is performed under anhydrous or semi anhydrous conditions by heating homogenised mixtures of polysaccharides and acidic monomers. Double bond modified polysaccharides and crosslinking of double bond modified polysaccharides have found some practical use is the preparation of medical gels (tissue engineering and drug delivery).
It has now been found that it is possible to thicken aqueous fluids for well treatments by addition of double bond modified polysaccharides and a radical polymerization initiator, possibly in the presence of other polyethylenically unsaturated compounds. The thickening is believed to be induced by radical self-crosslinking of the modified polysaccharides or by the radical crosslinking of the modified polysaccharides and the other ethylenically unsaturated compounds.
By “self-crosslinking” we mean the formation of covalent bonds between different polymeric chains of the modified polysaccharides.
By “crosslinking” we mean the formation of covalent bonds between at least one polymeric chain of a modified polysaccharides and at least another polyethylenically unsaturated compound (crosslinker).
The basic structure of the polysaccharide is not altered by the double bond formation and therefore the thickening of the aqueous fluid may be reversed by the breakers that are normally used in the field.